Conventionally, there has been known a so-called side reinforcing type run-flat tire provided with a reinforcing rubber layer in a side wall section. In accordance with the run-flat tire mentioned above, the reinforcing rubber layer supports a tire so as to suppress a flattening at a time when an air pressure in an inner section of the tire is lowered due to a puncture or the like, whereby a run-flat traveling can be executed. However, since a pressure application to a rim of a bead section is weakened and a fitting force is lowered in a state in which the air pressure in the inner section of the tire is lowered (in a run-flat state), there is a problem that the rim tends to be detached from the bead section.
As a countermeasure for the rim disengagement, there has been known a run-flat tire provided with an annular protrusion section in an outer side in a tire width direction of a bead section. For example, since an annular protrusion section 10 shown in FIG. 8(a) has a smaller inner diameter than an outer diameter of a rim flange while protruding to an outer side of the rim flange of a rim 8, it is possible to improve a rim disengagement resistance while holding the rim flange at a time of a run-flat traveling. However, in the run-flat tire mentioned above, it has been known that the rim disengagement resistance is not sufficient. In other words, as shown in FIG. 8(b), if a turning travel is executed at a comparatively high speed in a direction of R from a run-flat state, a side force F caused by a centrifugal force is applied to a vehicle so as to move the annular protrusion section 10 away from the rim flange. Accordingly, there is a problem that a bead section 1o positioned in an outer side of the vehicle tends to get over a hump of the rim 8 so as to disengage to an inner side.
On the contrary, as shown in FIG. 9, there has been proposed a so-called double bead type run-flat tire provided with an annular protrusion section 10 protruding to an outer side in a tire width direction of a bead section 1, and a second bead 1b arranged in the annular protrusion section 10. In accordance with the run-flat tire, since the annular protrusion section 10 reinforced by the second bead 1b can firmly hold a rim flange 8a at a time of a run-flat traveling, it is possible to effectively prevent the rim disengagement. The run-flat tire mentioned above is disclosed, for example, in the following patent documents 1 to 3.
However, in the conventional double bead type run-flat tire, it is hard to assemble the rim. In other words, in the rim assembly of the tire, it is necessary to temporarily drop down one side bead section to a concave section (a rim drop section) formed in a rim well section and thereafter install the bead section on a rim base. However, in the double bead type run-flat tire, it is necessary to install the bead section on the rim base and thereafter pull up the annular protrusion section to an outer side so as to get over the rim flange. As mentioned above, since the annular protrusion section has the inner diameter smaller than the outer diameter of the rim flange, and is reinforced by the second bead, it is very hard to execute an operation.
In other words, in the conventional double bead type run-flat tire, since the annular protrusion section provided for securing the rim disengagement resistance deteriorates a rim assembling property, it is hard to achieve both of the rim disengagement resistance and the rim assembling property. In this case, in the following patent document 4, there has been proposed a tire which can improve the rim disengagement resistance without deteriorating the rim assembling property, however, the tire mentioned above is structured such as to secure the rim assembling property by bending a flexible protrusion section to an outer side, and there is not disclosed a structure which can prevent a rim disengagement in the run-flat state by forming the second bead.
Further, in the conventional double bead type run-flat tire, there is a case that the rim disengagement resistance is not sufficient, and the conventional double bead type run-flat tire can not respond to a demand of intending to securely prevent the rim disengagement. In conventional, in order to improve the rim disengagement resistance, there have been employed a means for increasing a volume of the reinforcing rubber layer or increasing a tread rigidity, however, there has been a problem that a deterioration of ride comfort performance and an increase of a tire weight are caused.
The inventors of the present invention have devoted themselves to research on the problem of the rim disengagement mentioned above, and have found the following matters. FIG. 10 is a cross sectional view of a main section showing a bead section of a conventional double bead type run-flat tire. In an upper side of a bead section 1, there is shown a graph of a fitting pressure applied to a rim base 8b at a time of grounding in the run-flat traveling. As shown in FIG. 10, it has been known that a fitting pressure of a bead toe it is comparatively low at a time of the run-flat traveling, in the conventional run-flat tire. Further, the inventors of the present invention have found that the reduction of the fitting pressure of the bead toe it lowers a fitting force to a rim 8, and deteriorates the rim disengagement resistance.
In this case, the run-flat tire described in the following patent document 5 is structured such that a rim displacement preventing layer is provided in the bead reinforcing section for the purpose of increasing a traveling performance in the run-flat state, however, the patent document 5 does not disclose a structure which can solve the problem of the rim disengagement resistance as mentioned below.
Patent Document 1: Japanese Patent Application Laid-open No. S51-116507
Patent Document 2: Japanese Patent Application Laid-open No. S52-121204
Patent Document 3: Japanese Patent Application Laid-open No. S53-138106
Patent Document 4: Japanese Patent Application Laid-open No. 2000-6621
Patent Document 5: Japanese Patent Application Laid-open No. H11-157311